How to execute a command whenever a file changes?

entr (http://entrproject.org/) provides a more friendly interface to inotify (and also supports *BSD & Mac OS X).

It makes it very easy to specify multiple files to watch (limited only by ulimit -n), takes the hassle out of dealing with files being replaced, and requires less bash syntax:

$ find . -name '*.py' | entr ./myfile.py

I've been using it on my entire project source tree to run the unit tests for the code I'm currently modifying, and it's been a huge boost to my workflow already.

Flags like -c (clear the screen between runs) and -d (exit when a new file is added to a monitored directory) add even more flexibility, for example you can do:

$ while sleep 1 ; do find . -name '*.py' | entr -d ./myfile.py ; done

As of early 2018 it's still in active development and it can be found in Debian & Ubuntu (apt install entr); building from the author's repo was pain-free in any case.

I wrote a Python program to do exactly this called when-changed.

Usage is simple:

when-changed FILE COMMAND...

Or to watch multiple files:

when-changed FILE [FILE ...] -c COMMAND

FILE can be a directory. Watch recursively with -r. Use %f to pass the filename to the command.

How about this script? It uses the stat command to get the access time of a file and runs a command whenever there is a change in the access time (whenever file is accessed).

#!/bin/bash

### Set initial time of file
LTIME=`stat -c %Z /path/to/the/file.txt`

while true    
do
   ATIME=`stat -c %Z /path/to/the/file.txt`

   if [[ "$ATIME" != "$LTIME" ]]
   then    
       echo "RUN COMMAND"
       LTIME=$ATIME
   fi
   sleep 5
done

Solution using Vim:

:au BufWritePost myfile.py :silent !./myfile.py

But I don't want this solution because it's kinda annoying to type, it's a bit hard to remember what to type, exactly, and it's a bit difficult to undo its effects (need to run :au! BufWritePost myfile.py). In addition, this solution blocks Vim until the command has finished executing.

I've added this solution here just for completeness, as it might help other people.

To display the program output (and completely disrupt your editting flow, as the output will write over your editor for a few seconds, until you press Enter), remove the :silent command.

If you happen to have npm installed, nodemon is probably the easiest way to get started, especially on OS X, which apparently doesn't have inotify tools. It supports running a command when a folder changes.

rerun2 (on github) is a 10-line Bash script of the form:

#!/usr/bin/env bash

function execute() {
    clear
    echo "$@"
    eval "$@"
}

execute "$@"

inotifywait --quiet --recursive --monitor --event modify --format "%w%f" . \
| while read change; do
    execute "$@"
done

Save the github version as 'rerun' on your PATH, and invoke it using:

rerun COMMAND

It runs COMMAND every time there's a filesystem modify event within your current directory (recursive.)

Things one might like about it:

• It uses inotify, so is more responsive than polling. Fabulous for running sub-millisecond unit tests, or rendering graphviz dot files, every time you hit 'save'.
• Because it's so fast, you don't have to bother telling it to ignore large subdirs (like node_modules) just for performance reasons.
• It's extra super responsive, because it only calls inotifywait once, on startup, instead of running it, and incurring the expensive hit of establishing watches, on every iteration.
• It's just 12 lines of Bash
• Because it's Bash, it interprets commands you pass it exactly as if you had typed them at a Bash prompt. (Presumably this is less cool if you use another shell.)
• It doesn't lose events that happen while COMMAND is executing, unlike most of the other inotify solutions on this page.
• On the first event, it enters a 'dead period' for 0.15 seconds, during which other events are ignored, before COMMAND is run exactly once. This is so that the flurry of events caused by the create-write-move dance which Vi or Emacs does when saving a buffer don't cause multiple laborious executions of a possibly slow-running test suite. Any events which then occur while COMMAND is executing are not ignored - they will cause a second dead period and subsequent execution.

Things one might dislike about it:

• It uses inotify, so won't work outside of Linuxland.
• Because it uses inotify, it will barf on trying to watch directories containing more files than the max number of user inotify watches. By default, this seems to be set to around 5,000 to 8,000 on different machines I use, but is easy to increase. See https://unix.stackexchange.com/questions/13751/kernel-inotify-watch-limit-reached
• It fails to execute commands containing Bash aliases. I could swear that this used to work. In principle, because this is Bash, not executing COMMAND in a subshell, I'd expect this to work. I'd love to hear If anyone knows why it doesn't. Many of the other solutions on this page can't execute such commands either.
• Personally I wish I was able to hit a key in the terminal it's running in to manually cause an extra execution of COMMAND. Could I add this somehow, simply? A concurrently running 'while read -n1' loop which also calls execute?
• Right now I've coded it to clear the terminal and print the executed COMMAND on each iteration. Some folks might like to add command-line flags to turn things like this off, etc. But this would increase size and complexity many-fold.

This is a refinement of @cychoi's anwer.

Here's a simple shell Bourne shell script that:

1. Takes two arguments: the file to be monitored and a command (with arguments, if necessary)
2. Copies the file you are monitoring to the /tmp directory
3. Checks every two seconds to see if the file you are monitoring is newer than the copy
4. If it's newer it overwrites the copy with the newer original and executes the command

5. Cleans up after itself when you press Ctr-C

   #!/bin/sh  
   f=$1  
   shift  
   cmd=$*  
   tmpf="`mktemp /tmp/onchange.XXXXX`"  
   cp "$f" "$tmpf"  
   trap "rm $tmpf; exit 1" 2  
   while : ; do  
       if [ "$f" -nt "$tmpf" ]; then  
           cp "$f" "$tmpf"  
           $cmd  
       fi  
       sleep 2  
   done  
   

This works on FreeBSD. The only portability issue I can think of is if some other Unix doesn't have the mktemp(1) command, but in that case you can just hard code the temp file name.

For those who can't install inotify-tools like me, this should be useful:

watch -d -t -g ls -lR

This command will exit when the output changes, ls -lR will list every file and directory with its size and dates, so if a file is changed it should exit the command, as man says:

-g, --chgexit
          Exit when the output of command changes.

I know this answer may not be read by anyone, but I hope someone would reach to it.

Command line example:

~ $ cd /tmp
~ $ watch -d -t -g ls -lR && echo "1,2,3"

Open another terminal:

~ $ echo "testing" > /tmp/test

Now the first terminal will output 1,2,3

Simple script example:

#!/bin/bash
DIR_TO_WATCH=${1}
COMMAND=${2}

watch -d -t -g ls -lR ${DIR_TO_WATCH} && ${COMMAND}

Have a look at incron. It's similar to cron, but uses inotify events instead of time.

Another solution with NodeJs, fsmonitor :

1. Install

   sudo npm install -g fsmonitor
   

2. From command line (example, monitor logs and "retail" if one log file change)

   fsmonitor -s -p '+*.log' sh -c "clear; tail -q *.log"
   

Under Linux:

man watch

watch -n 2 your_command_to_run

Will run the command every 2 seconds.

If your command takes more than 2 seconds to run, watch will wait until it's done before doing it again.

Look into Guard, in particular with this plugin:

https://github.com/hawx/guard-shell

You can set it up to watch any number of patterns in your project's directory, and execute commands when changes occur. Good chance even that there's a plugin available for that what you're trying to do in the first place.

if you have nodemon installed, then you can do this:

nodemon -w <watch directory> -x "<shell command>" -e ".html"

In my case I edit html locally and ship it to my remote server when a file changes.

nodemon -w <watch directory> -x "scp filename jaym@jay-remote.com:/var/www" -e ".html"

If your program generates some sort of log/output, you can create a Makefile with a rule for that log/output that depends on your script and do something like

while true; do make -s my_target; sleep 1; done

Alternately, you can create a phony target and have the rule for it both call your script and touch the phony target (while still depending on your script).

swarminglogic wrote a script called watchfile.sh, also available as a GitHub Gist.

Improved upon Gilles's answer.

This version runs inotifywait once and monitors for events (.e.g.: modify) thereafter. Such that inotifywait doesn't need to be re-executed upon every event encountered.

It's quick and fast!_{(even when monitoring large directory recursively)}

inotifywait --quiet --monitor --event modify FILE | while read; do
    # trim the trailing space from inotifywait output
    REPLY=${REPLY% }
    filename=${REPLY%% *}
    # do whatever you want with the $filename
done

A little more on the programming side, but you want something like inotify. There are implementations in many languages, such as jnotify and pyinotify.

This library allows you to monitor single files or entire directories, and returns events when an action is discovered. The information returned includes the file name, the action (create, modify, rename, delete) and the file path, among other useful information.

For those of you who are looking for a FreeBSD solution, here is the port:

/usr/ports/sysutils/wait_on

I like the simplicity of while inotifywait ...; do ...; done however it has two issues:

• File changes happening during the do ...; will be missed
• Slow when using in recursive mode

Therefor I made a helper script that uses inotifywait without those limitations: inotifyexec

I suggest you put this script in your path, like in ~/bin/. Usage is described by just running the command.

Example: inotifyexec "echo test" -r .

Watchdog is a Python project, and may be just what you're looking for:

Supported platforms

• Linux 2.6 (inotify)
• Mac OS X (FSEvents, kqueue)
• FreeBSD/BSD (kqueue)
• Windows (ReadDirectoryChangesW with I/O completion ports; ReadDirectoryChangesW worker threads)
• OS-independent (polling the disk for directory snapshots and comparing them periodically; slow and not recommended)

Just wrote a command-line wrapper for it watchdog_exec:

Example runs

On fs event involving files and folders in current directory, run echo $src $dst command, unless it the fs event is modified, then run python $src command.

python -m watchdog_exec . --execute echo --modified python

Using short arguments, and restricting to only execute when events involve "main.py":

python -m watchdog_exec . -e echo -a echo -s __main__.py

EDIT: Just found Watchdog has an official CLI called watchmedo, so check that out also.

Improved Sebastian's solution with watch command:

watch_cmd.sh:

#!/bin/bash
WATCH_COMMAND=${1}
COMMAND=${2}

while true; do
  watch -d -g "${WATCH_COMMAND}"
  ${COMMAND}
  sleep 1     # to allow break script by Ctrl+c
done

Call example:

watch_cmd.sh "ls -lR /etc/nginx | grep .conf$" "sudo service nginx reload"

It works but be careful: watch command has known bugs (see man): it reacts on changes only in VISIBLE in terminal parts of -g CMD output.